Monovalent Selective Cation Exchange Membrane

ABSTRACT

A monovalent selective ion exchange membrane is disclosed. The membrane includes a polymeric microporous substrate, a cross-linked ion-transferring polymeric layer on a surface of the substrate, and a charged functionalizing layer covalently bound to the ion-transferring layer. A method of producing a monovalent selective cation exchange membrane is also disclosed. The method may include chemically adsorbing a styrene intermediate layer to a cross-linked ion-transferring polymeric layer on a surface of a polymeric microporous substrate, chlorosulfonating the styrene intermediate layer to attach a sulfonyl chloride group layer, aminating the sulfonyl group layer to attach an amine group layer, and functionalizing the amine group layer with a charged compound layer to produce the cation exchange membrane. Water treatment systems including the monovalent selective cation exchange membrane and methods of facilitating water treatment including providing the monovalent selective cation exchange membrane are also disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. § 119(e) to U.S. Provisional Application Ser. No. 62/737,373 titled “Monovalent Selective Cation Exchange Membrane” filed Sep. 27, 2018, U.S. Provisional Application Ser. No. 62/736,176 titled “Cation Exchange Membrane Through UV Initiated Polymerization” filed Sep. 25, 2018, and U.S. Provisional Application Ser. No. 62/861,608 titled “Exchange Membrane Preparation by UV Light Polymerization” filed Jun. 14, 2019, each of which is incorporated herein by reference in its entirety for all purposes.

FIELD OF TECHNOLOGY

Aspects and embodiments disclosed herein are generally related to ion exchange membranes, and more specifically, to monovalent selective ion exchange membranes.

SUMMARY

In accordance with one aspect, there is provided a monovalent selective ion exchange membrane. The monovalent selective ion exchange membrane may comprise a polymeric microporous substrate. The monovalent selective ion exchange membrane may comprise a cross-linked ion-transferring polymeric layer on a surface of the substrate. The monovalent selective ion exchange membrane may comprise a charged functionalizing layer covalently bound to the cross-linked ion-transferring polymeric layer.

In some embodiments, the membrane may have a total thickness of about 20 μm to about 155 μm. the membrane may have a total thickness of about 25 μm to about 55 μm.

The monovalent selective ion exchange membrane may be a cation exchange membrane. The charged functionalizing layer may be a positively charged functionalizing layer.

In some embodiments, the positively charged functionalizing layer may comprise at least one of a sulfonic acid group, a carboxylic acid group, a quaternary ammonium, and a tertiary amine group hydrolyzed into a positively charged ammonium.

The monovalent selective membrane may be an anion exchange membrane. The charged functionalizing layer may be a negatively charged functionalizing layer.

The monovalent selective ion exchange membrane may have a counter ion permselectivity of at least 100%.

The monovalent selective ion exchange membrane may have an initial selectivity of 8 to 12 fold Na/Ca (ppm) at room temperature.

The monovalent selective membrane may have a resistivity of less than about 5 Ω-cm².

The polymeric microporous substrate may comprise at least one of high-density polyethylene (HDPE) and ultrahigh molecular weight polyethylene (UHMWPE).

In accordance with another aspect, there is provided a monovalent selective cation exchange membrane support. The monovalent selective cation exchange membrane support may comprise a polymeric microporous substrate. The monovalent selective cation exchange membrane support may comprise a cross-linked ion-transferring polymeric layer on a surface of the substrate. The monovalent selective cation exchange membrane support may comprise an intermediate layer comprising an amine group covalently bound to the cross-linked ion-transferring polymeric layer.

In some embodiments, the intermediate layer may comprise a primary amine group or a secondary amine group.

The intermediate layer may comprise polyethylenimine (PEI).

The intermediate layer may comprise a branched PEI having a molecular weight of at least 600 g/mol.

The intermediate layer may be covalently bound to the cross-linked ion-transferring polymeric layer by a styrene group.

The styrene group may be chemically bound to chlorosulfonated divinylbenzene (DVB).

In accordance with another aspect, there is provided a method of producing a monovalent selective cation exchange membrane. The method may comprise chemically adsorbing a styrene intermediate layer to a cross-linked ion-transferring polymeric layer on a surface of a polymeric microporous substrate. The method may comprise chlorosulfonating the styrene intermediate layer to attach a sulfonyl chloride group layer to the surface of the polymeric microporous substrate. The method may comprise aminating the sulfonyl group layer to attach an amine group layer to the surface of the polymeric microporous substrate. The method may comprise functionalizing the amine group layer with a charged compound layer to produce the monovalent selective cation exchange membrane.

In some embodiments, the method may comprise chemically adsorbing styrene DVB to the cross-linked ion-transferring polymeric layer on a surface of a polymeric microporous substrate.

The method may comprise chlorosulfonating the styrene DVB with chlorosulfonic acid (ClSO₃H) to attach a ClSO₂ group to the styrene DVB.

The method may comprise aminating the ClSO₂ with PEI.

The method may comprise aminating the ClSO₂ with branched PEI having a molecular weight of at least 600 g/mol.

The method may comprise functionalizing the amine group layer with a positively charged group.

The method may comprise functionalizing the amine group layer with a positively charged ammonium.

The method may further comprise soaking the polymeric microporous substrate with a solution comprising an ionogenic monomer, a multifunctional monomer, and a polymerization initiator to produce the cross-linked ion-transferring polymeric layer.

In accordance with another aspect, there is provided a water treatment system. The water treatment system may comprise a source of water to be treated. The water treatment system may comprise an electrochemical separation device fluidly connected to the source of water to be treated and comprising at least one monovalent selective cation exchange membrane having a charged functionalizing layer covalently bound to a surface of the cation exchange membrane. The water treatment system may comprise a treated water outlet fluidly connected to the electrochemical separation device.

In some embodiments, the source of water to be treated may comprise at least one hardness ion selected from Ca²⁺ and Mg²⁺.

In some embodiments, the charged functionalizing layer may be a positively charged functionalizing layer comprising at least one of a sulfonic acid group, a carboxylic acid group, quaternary ammonium, and a tertiary amine group hydrolyzed into a positively charged ammonium.

In some embodiments, the charged functionalizing layer may be covalently bound to the surface of the cation exchange membrane by a chemically adsorbed branched PEI layer.

In accordance with another aspect, there is provided a method of facilitating water treatment with an electrochemical separation device. The method may comprise providing a monovalent selective cation exchange membrane having a charged functionalizing layer covalently bound to a surface of the cation exchange membrane. The method may comprise instructing a user to install the monovalent selective cation exchange membrane in the electrochemical separation device.

In some embodiments, the method may comprise instructing the user to fluidly connect the electrochemical separation device to a source of water to be treated comprising at least one hardness ion selected from Ca²⁺ and Mg²⁺.

The method may comprise providing a monovalent selective cation exchange membrane support having a polymeric microporous substrate with an amine group layer covalently bound to a surface of the polymeric microporous substrate. The method may further comprise instructing a user to functionalize the amine group layer with a charged compound layer to produce the cation exchange membrane.

In accordance with yet another aspect, there is provided a monovalent selective cation exchange membrane. The cation exchange membrane may comprise a polymeric microporous substrate and a positively charged functionalizing layer covalently bound to a surface of the polymeric microporous substrate. The monovalent selective cation exchange membrane may have an initial selectivity of 8 to 12 fold Na/Ca (ppm) at room temperature.

The monovalent selective cation exchange membrane may have a selectivity of 4 to 8 fold Na/Ca (ppm) after 400 days in 0.5M NaCl at room temperature.

The monovalent selective cation exchange membrane may have an initial selectivity of 10 to 40 fold Na/Ca (molar) at 80° C.

The monovalent selective cation exchange membrane may have a selectivity of 3 to 6 fold Na/Ca after (molar) 30 days in 0.5M NaCl at 80° C.

The disclosure contemplates all combinations of any one or more of the foregoing aspects and/or embodiments, as well as combinations with any one or more of the embodiments set forth in the detailed description and any examples.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are not intended to be drawn to scale. In the drawings, each identical or nearly identical component that is illustrated in various figures is represented by a like numeral. For purposes of clarity, not every component may be labeled in every drawing. In the drawings:

FIG. 1 is a representation of the chemical structure of the polyethylenimine (PEI) molecule showing primary (—NH2), secondary (—NH—), and tertiary amine groups;

FIG. 2 is a representation of the formation of an ionic bond by physiosorption between a primary or secondary amine of PEI and a sulfonic acid group of a cation exchange membrane surface, according to one embodiment;

FIG. 3 is a representation of the formation of a covalent bond by chemisorption between a primary or secondary amine of PEI and chlorosulfonic acid (ClSO₃H), which occurs as a two-step process, according to one embodiment;

FIG. 4A is a graph of concentration of Ca²⁺ and Na in a dilute stream over time for water treatment with a conventional membrane;

FIG. 4B is a graph of concentration of Ca²⁺ and Na in a dilute stream over time for water treatment with an alternate conventional membrane;

FIG. 5 is a graph of concentration of Ca²⁺ and Na in a dilute stream over time for water treatment with a monovalent selective ion exchange membrane, according to one embodiment;

FIG. 6A is a graph of ion concentration and sodium absorption rate (SAR) value of experimental ground water treated with a monovalent selective ion exchange membrane, according to one embodiment;

FIG. 6B is a graph of ion concentration and SAR value of experimental ground water treated with a conventional cation exchange membrane;

FIG. 7 is a graph of ion concentration in experimental seawater treated with a monovalent selective ion exchange membrane, according to one embodiment;

FIG. 8 is a graph of monovalent transport selectivity over time for water treatment with a monovalent selective ion exchange membrane, according to one embodiment;

FIG. 9 is a schematic diagram of a membrane selectivity experimental apparatus;

FIG. 10A is a graph showing the concentration of target cations in a dilute stream desalted by a monovalent selective cation exchange membrane, according to one embodiment;

FIG. 10B is a graph showing the concentration of target cations in a dilute stream desalted by a conventional cation exchange membrane;

FIG. 10C is a graph showing the concentration of target cations in a dilute stream produced by a monovalent selective cation exchange membrane, according to one embodiment;

FIG. 10D is a graph showing the concentration of target cations in a dilute stream produced by a conventional cation exchange membrane;

FIG. 11A is a graph of the concentration of select ions in the concentrate compartment using a monovalent selective anion exchange membrane for treatment of seawater with an applied density of 300 A/m², according to one embodiment;

FIG. 11B is a graph of the concentration of select ions in the concentrate compartment using monovalent selective cation exchange membrane for treatment of seawater with an applied current density of 300 A/m²; according to one embodiment;

FIG. 12A is a graph showing the lifetime selectivity (stability) of a conventional/commercially available monovalent selective membrane and a monovalent selective membrane disclosed herein at 80° C., according to one embodiment; and

FIG. 12B is a graph showing the lifetime selectivity (stability) of a conventional/commercially available monovalent selective membrane and a monovalent selective membrane disclosed herein at room temperature, according to one embodiment.

DETAILED DESCRIPTION

Embodiments disclosed herein provide for ion exchange membranes and processes for their manufacture. The electrodialysis (ED) membranes described herein may generally combine low resistance and high permselectivity. Their properties may make them highly effective in water desalination applications, particularly in seawater desalination. Their properties make them highly effective in treatment of irrigation water, particularly for adjustment of sodium absorption rate (SAR) value. The ion exchange membranes described herein may be manufactured by polymerizing one or more monofunctional ionogenic monomers, optionally a neutral monomer with at least one multifunctional monomer, in the pores of a porous substrate.

Ion exchange membranes are typically employed to transport cations or anions under an electrical or chemical potential. Ion exchange membranes may have either negatively or positively charged groups attached to the polymeric material making up the bulk of the membrane. The counterion of each group typically functions as the transferable ion. A cation exchange membrane may have fixed negative charges and mobile positively charged cations. An anion exchange membrane may have fixed positively charged groups and mobile negatively charged anions. Ion exchange membrane properties may be engineered by controlling the amount, type, and distribution of the fixed ionic groups. These membranes may be described as strong acid, strong base, weak acid, or weak base membranes. Strong acid cation exchange membranes typically have sulfonic acid groups as the charged group. Weak acid membranes typically have carboxylic acid groups making up the fixed charged group. Quaternary and tertiary positively charged ammonium, respectively, may produce the fixed positive charged groups in strong and weak base anion exchange membranes.

Ion exchange membranes may be used for desalination of water by electrodialysis (ED), as a power generating source in reverse electrodialysis, or as separators in fuels cells. Thus, water treatment systems disclosed herein may be or comprise desalination systems, power generating systems, or reverse electrodialysis systems. Other applications include recovery of metal ions in the electroplating and metal finishing industries and applications in the food and beverage industry. In other embodiments, water treatment systems disclosed herein may be or comprise metal ion recovery systems or food and beverage processing systems.

In a particular exemplary embodiment, ion exchange membranes disclosed herein may be used for ground water treatment and/or in agricultural settings. The water treatment systems disclosed herein may be or comprise ground water treatment systems. The water treatment systems disclosed herein may be or comprise agricultural irrigation runoff treatment systems. The methods may comprise treating ground water. The methods may comprise treating agricultural water runoff.

Electrodialysis generally desalinates water by transferring ions and some charged organics through paired anion- and cation selective membranes under the motive force of a direct current voltage. An ED apparatus may include electrically conductive and substantially water impermeable anion selective and cation selective membranes arranged as opposing walls of a cell. Adjacent cells typically form a cell pair. Membrane stacks may include many, sometime hundreds, of cell pairs. An ED system may include many stacks. Each membrane stack typically has a DC (direct current) anode at one end of the stack and a DC cathode at the other end. Under a DC voltage, ions may move toward the electrode of opposite charge.

A cell pair includes two types of cells, diluting cells and concentrating cells. Each type of cell may be defined by opposing membranes. One exemplary cell pair may include a common cation transfer membrane wall and two anion transfer membrane walls forming the two cells. That is, a first anion transfer membrane and the cation transfer membrane form the diluting cell, and the cation transfer membrane and a second anion transfer membrane form the concentrating cell. In the diluting cell, cations typically pass through the cation transfer membrane facing the anode, but may be stopped by the paired anion transfer membrane of the concentrating cell in that direction facing the cathode. Similarly, anions may pass through the anion transfer membrane of the diluting cell facing the cathode, but may be stopped by the cation transfer membrane of the adjacent pair facing the anode. In this manner, salt in a diluting cell may be removed. In the adjacent concentrating cell, cations may enter from one direction and anions from the opposite direction. Flow in the stack may be arranged so that the dilute and concentrated flows are kept separate. Thus a desalinated water stream may be produced from the dilute flow.

Scarcity of irrigation water of sufficient quality is deleterious to crop yields and may require choice of crop species that are of less demand. Newer methods of irrigation that reduce the amount of water used, using techniques such as drip irrigation, may also cause a non-sustainable condition due to salt and impurity buildup in the soil from the water used for irrigation. The soil salinity may rise to much higher concentrations than in the irrigation water due to use of most of the water by the crops, and by evaporation. Conditions of irrigation and soil with inadequate source water for leaching the soil or insufficient rainfall may result in soil salinities 4 to 5 times higher than in the irrigation water itself. Further, should the land consist of relatively shallow impermeable ground layers, the irrigation water may raise the water table. When highly saline ground water reaches crop root levels, the water may be harmful to crop growth. Also, saline soils may damage leafy crops due to water splash off the soil surface. Furthermore, if the agricultural land is drained of the saline water, trace impurities in the soil such as selenium or boron, or residual contaminants from fertilizer use such as nitrate may cause contamination of the drainage water and cause difficulties in safe effluent control.

When irrigating crops, the yield may be affected by total dissolved salts (TDS) concentration. The TDS is typically correlated to conductivity value. For example, a conductivity value of 1 mS/cm corresponds to approximately 500-700 ppm TDS. Various plants benefit from low TDS irrigation water. For example, bean, carrots, and strawberries may benefit from irrigation with water having a conductivity lower than 1 mS/cm. Other plants may tolerate irrigation water with a conductivity of about 5 mS/cm. Furthermore, control of the SAR value at a given TDS and conductivity may affect soil flocculation and efficient water infiltration. For instance, irrigation water having a conductivity of less than 1 mS/cm may benefit from a SAR value of greater than 3 to maintain soil structure. Irrigation water having a conductivity of 2-3 mS/cm may benefit from a SAR value of about 10.

Irrigation water needs also are in competition with potable drinking water for humans, and water free of contaminants for livestock, and wildlife. Thus, it is commonly the case that a source of a combination of irrigation water and potable water are needed in agricultural regions. The membranes described herein may be employed for agricultural irrigation water treatment. In particular, the membranes described herein may be employed to control TDS, conductivity, and SAR value for agricultural irrigation water. In some embodiments, the membranes described herein may provide water having a conductivity of less than 1 mS/cm. The membranes described herein may provide water having a conductivity of between 2-3 mS/cm, between 3-5 mS/cm, or greater than 5 mS/cm (for example, between 5-7 mS/cm). The membranes described herein may provide water having a SAR value of greater than 3, for example, between 3-5. The membranes described herein may provide water having a SAR value of greater than 5, for example, between 5-10. The membranes described herein may provide water having a SAR value of about 10 or greater, for example, between 10-12.

Univalent selective or monovalent selective membranes primarily transfer monovalent ions. Monovalent selective membranes may separate ions on the basis of charge and/or size. Monovalent selective membranes may distinguish between monovalent and divalent ions. Monovalent selective cation transfer membranes may distinguish between ions having a charge of +1, for example, sodium and potassium, and ions having a greater positive charge, for example, magnesium and calcium. Thus, monovalent selective cation exchange membranes described herein may selectively transport monovalent ions such as sodium and potassium ions, while blocking transport of divalent ions such as calcium and magnesium ions. Similarly, monovalent selective anion membranes may separate ions having a charge of −1, such as chloride, bromide, and nitrate, from ions having a greater negative charge. Thus, monovalent anion exchange membranes described herein may selectively transport monovalent ions such as chloride and nitrate ions, while blocking transport of divalent ions such as sulfate ions.

The ion exchange membranes disclosed herein may be used to treat brackish water and waste water desalination. Even though ED is generally considered too expensive for seawater use, the ion exchange membranes disclosed herein may be used efficiently for seawater desalination. Effective and efficient seawater desalination may be performed with a membrane resistance of less than 1 Ω-cm², for example, less than 0.8 Ω-cm², or less than 0.5Ω-cm². The ion exchange membranes disclosed herein may also provide an ion permselectivity of greater than 90%, for example, greater than 95%, or greater than 98%. Additionally, the ion exchange membranes disclosed herein have a longer service life and greater physical strength and chemical durability than comparable conventional ion exchange membranes. Finally, the ion exchange membranes disclosed herein may be manufactured at a comparatively low cost.

As a result, the ion exchange membranes disclosed herein may be employed in reverse electrodialysis (RED). RED may be used to convert free energy generated by mixing two aqueous solutions of different salinities into electrical power. In general, the greater the difference in salinity, the greater the potential for power generation. The water treatment systems disclosed herein may be or comprise RED systems. The methods disclosed herein may be employed to generate electrical power.

The ion exchange membranes disclosed herein may be employed as a polymer electrolyte membrane (PEM). A PEM is a type of ion exchange membrane that may serve both as the electrolyte and as a separator to prevent direct physical mixing of the hydrogen from the anode and oxygen supplied to the cathode. A PEM may contain negatively charged groups, such as, sulfonic acid groups, attached or as part of the polymer making up the PEM. Protons typically migrate through the membrane by jumping from one fixed negative charge to another to permeate the membrane.

The membranes disclosed herein may generally comprise an ion exchange membrane support and a charged functionalizing layer covalently bound to the ion exchange membrane support. The ion exchange membrane support may comprise a polymeric microporous substrate and a cross-linked ion-transferring polymeric layer on a surface of the substrate. As an intermediate production step, the membrane support may additionally comprise an amine group layer covalently bound to the cross-linked ion-transferring polymeric layer.

The membranes described herein may generally exhibit good mechanical strength. The mechanical strength may be sufficient to allow the membrane to withstand the stresses of a continuous membrane manufacturing process, and be fabricated and sealed into the final membrane-holding device or module without overt damage or hidden damage which could appear after some time of operation. In addition, the mechanical strength may be sufficient to provide high dimensional stability. The membrane may generally exhibit minimal variation in dimensions while working as a desalination apparatus, during cleaning, sanitizing or defouling regimes, or during shipping or while in storage. High dimensional stability to changes in ionic content or temperature, for example, of the fluid contacting the membrane, may be provided, such that during operation variations in the distance between membrane pairs which could lead to current inefficiencies are minimized. Changes in dimensions during electrodialysis which could cause stresses in the constrained membrane leading to membrane defects and poor performance, may also generally be minimized.

The membranes described herein may exhibit low resistance. In general, low resistance reduces the electrical energy required to desalinate and lowers operating cost. Specific membrane resistance is sometimes measured in Ω-cm. Another engineering measure is Ω-cm². Resistance may be measured by a resistance testing process which uses a cell having two electrodes of known area in an electrolyte solution. Platinum or black graphite are typically used for the electrodes. Resistance is then measured between the electrodes. A membrane sample of known area may be positioned between the electrodes in the electrolyte solution. The electrodes do not touch the membrane. Resistance is then measured again with the membrane in place. Membrane resistance may then be estimated by subtracting the electrolyte resistance without the membrane from the test result with the membrane in place.

The resistance may also be measured by determining a voltage vs. current curve in a cell having two well stirred chambers separated by the membrane. A calomel electrode may be used to measure the potential drop across the membrane. The slope of the potential drop vs. current curves may be obtained by varying voltage and measuring current.

Electrochemical impedance may also be used for the calculation. In this method, alternating current may be applied across the membrane. Measurement at a single frequency gives data relating to electrochemical properties of the membrane. By using frequency and amplitude variations, detailed structural information may be obtained.

The membranes described herein may have high counter ion permselectivity. Permselectivity may generally refer to the relative transport of counter ions to co-ions during electrodialysis. For a theoretically ideal cation exchange membrane only positively charged ions would pass the membrane, giving a counter ion permselectivity of 1.0 or 100%. Permselectivity may be found by measuring the potential across the membrane while it separates monovalent salt solutions of different concentrations.

The ion exchange membranes disclosed herein may have reduced water permeation. Permeation of the dilute flow through membrane defects under the driving force of the osmotic pressure difference between the dilute and concentrated streams may reduce efficiency. Water permeation may reduce current efficiency and purified water productivity by removing pure water. Water loss may be particularly severe in seawater electrodialysis with thin membranes because the high concentration difference between the concentrate (brine) side of the membranes and the pure water side of the membrane typically increases the osmotic driving force. Membrane defects may be particularly detrimental to operation as the high osmotic pressure will tend to force pure water through such defects and increase water loss, increasing power consumption.

The membranes disclosed herein may generally have a structure that allows high permeability of cations and low osmotic flow. Apparent counter ion permselectivity as used herein is the ratio of transport rate of counter-ions (cations) to co-ions (anions). Conventional measurement parameters do not indicate the rate of counter-ion removal. In certain embodiments, the membranes disclosed herein may be engineered to control cation permeability.

Cation permeability may be controlled by the structure of the ion (molecular size and total charge) and by the effect of membrane microstructure. The membrane microstructure can retard counter-ion permeability if the membrane is designed to have pores that are comparatively small. The relative term can be taken to mean that the counter-ions encounter high resistance from interactions with the membrane material in traversing the membrane, as if they were traversing a tunnel slightly larger than their apparent diameter. The membrane may have a relatively low water content, tending to reduce the pathways for counter-ion permeability. By balancing the content of hydrophilic monomer to increase counter-ion permeability with the amount and nature of cross-linking monomer, the water content and effective pore size of the membrane can be engineered. The cross-linking monomer may be selected to be a hydrophobic or hydrophilic monomer.

The membranes disclosed herein may generally comprise an ion exchange membrane support. The ion exchange membrane support may comprise a polymeric microporous substrate and a cross-linked ion-transferring polymeric layer on a surface of the substrate. The membrane support may be produced by a process comprising selecting a suitable porous substrate and incorporating a cross-linked ion-transferring polymeric layer on a surface of the substrate.

The microporous membrane substrate may be manufactured from polyolefins, polyvinylidene fluoride, or other polymers. One exemplary class of substrates comprises thin polyolefin membranes. Another exemplary class of substrate are manufactured from high-density polyethylene (HDPE). Another exemplary class of substrates are manufactured from ultrahigh molecular weight polyethylene (UHMWPE). The microporous substrate may comprise microporous membranes of polypropylene, high molecular weight polyethylene, ultrahigh molecular weight polyethylene or polyvinylidene fluoride. The substrate may generally have a thickness of less than about 155 μm, for example, less than about 55 μm or less than about 25 μm.

The exemplary microporous membrane materials may be employed to manufacture very thin ion exchange membranes, for example, as disclosed in U.S. Pat. No. 8,703,831, incorporated herein by reference in its entirety for all purposes. The exemplary ion exchange membranes may have a thickness of 12-100 μm, for example, 25-32 μm. The thin membrane enables a fast chlorosulfonation reaction, described in more detail below, effective throughout a bulk of the membrane. For example, a 5 minute chlorosulfonation reaction may be sufficient to complete bulk chlorosulfonation. Thus, methods described herein may comprise performing a chlorosulfonation reaction effective to chlorosulfonate a bulk of the membrane in about 5 minutes.

Additionally, certain exemplary microporous membrane materials may be employed to provide stability against aggressive chemicals. For example, a membrane substrate of HDPE is generally stable against ClSO₃H. A material such as polypropylene may not be sufficiently stable against ClSO₃H.

Embodiments of the substrate membrane may have a porosity greater than about 45%, for example, greater than about 60%. In certain embodiments, the substrate membrane may have a porosity greater than about 70%. The substrate membrane may have a rated pore size of from about approximately 0.05 μm to about approximately 10 μm, for example, from about approximately 0.1 μm to about approximately 1.0 μm, or from about approximately 0.1 μm to about approximately 0.2 μm.

The membrane support may be produced by saturating the monomer solution in the pores of the substrate. The monomer solution may be polymerized from functional monomers, a cross-linking agent, and a polymerization initiator in the pores to form the cross-linked charged polymer. In certain embodiments, the functional monomers may include an ionogenic monomer, for example, a monofunctional ionogenic monomer, and a multifunctional monomer, for example, a cross-linking agent. As used herein, the term ionogenic monomer may generally refer to a monomer species having at least one charged group covalently attached. The charged group may be positively charged or negatively charged, as described in more detail below. Monofunctional monomers may generally refer to monomers which have a single site for carrying forward the polymerization reaction. Multifunctional monomers may generally refer to monomers that have more than one polymerization reaction site and so can form networked or crosslinked polymers.

The process of polymerizing the cross-linked ion-transferring polymeric layer in the pores of the substrate may include saturating the substrate with a solution comprising the monofunctional ionogenic monomer, the multifunctional monomer, and the polymerization initiator. The process may include removing excess solution from the surfaces of the substrate while leaving the porous volume saturated with solution and initiating polymerization. Polymerization may be initiated by the application of heat, ultraviolet (UV) light, or ionizing radiation, optionally in the absence of substantially all oxygen. The process may be performed to incorporate the cross-linked ion-transferring polymeric layer substantially completely filling the pores of the substrate.

Thus, in certain embodiments, the membrane support may be produced by the polymerization of one or more ionogenic monomers, a neutral monomer, and a suitable crosslinker monomer. Exemplary neutral monomers are hydroxyethyl acrylate and hydroxymethylmetacrylate. Other neutral monomers are within the scope of the disclosure. The ionogenic monomer may be selected to produce a cation exchange membrane or an anion exchange membrane.

Monomers containing negatively charged groups include as representative examples, without being limited by such examples, sulfonated acrylic monomers suitable to provide cation exchange capacity, for example, 2-sulfoethylmethacrylate (2-SEM), 2-Propylacrylic acid, 2-acrylamide-2-methyl propane sulfonic acid (AMPS), sulfonated glycidylmethacrylate, 3-sulfopropyl methacrylate, sodium 1-allyloxy-2 hydroxypropyl sulfonate and the like. Other exemplary monomers are acrylic and methacrylic acid or their salts, sodium styrene sulfonate, styrene sulfonic acid, sulfonated vinylbenzyl chloride sodium 1-allyloxy-2 hydroxypropyl sulfonate, 4-Vinylbenzoic acid, Trichloroacrylic acid, vinyl phosphoric acid and vinyl sulfonic acid. Preferred monomers are 2-sulfoethylmethacrylate (2-SEM), styrene sulfonic acid and its salts, and 2-acrylamide-2-methyl propane sulfonic acid (AMPS).

Cation exchange membrane embodiments described herein may have a resistivity of less than about approximately 1.0 Ω-cm², for example, less than about approximately 0.5 S2-cm². Certain embodiments of the cation exchange membranes described herein may have a permselectivity of greater than about approximately 95%, for example, greater than about approximately 99%. In some embodiments, the ionogenic monomers for the production of cation exchange membranes may be or comprise 2-sulfoethylmethacrylate (2-SEM or 2-acrylamide-2-methyl propane sulfonic acid (AMPS). One exemplary cross-linker is ethyleneglycoldimethacrylate. Other ionogenic monomers and crosslinkers are within the scope of the disclosure.

Monomers containing positively charged groups include as representative examples, without being limited by such examples, Methacrylamidopropyltrimethyl ammonium chloride, trimethylammoniumethylmethacrylate, quaternary salts of polyamines and vinylaromatic halides, for example, 1,4-diazabicyclo[2,2,2]octane di(vinylbenzyl chloride) (a quaternary salt of 1,4-diazabicyclo[2,2,2]octane (DABCO) and piperazine divinyl chloride), or quaternary salts formed by reacting cyclic ethers, polyamines and alkyl halides, for example, Iodoethyldimethylethylenediamino 2-hydroxylpropyl methacrylate (a quaternary ammonium salt formed by reacting glycidylmethacrylate (GMA) with N,N-dimethylethylenediamine and ethyl iodide), and vinylbenyltrimethylammonium chloride. Other exemplary monomers for anion exchange membranes include Trimethylammoniumethylmethacrylic chloride, 3-(acrylamidopropyl)trimethylammonium chloride, N,N,N′,N′,N″-pentamethyldiethylenetriamine di(vinylbenzyl chloride (a quaternary salt of N,N,N′,N′,N″-pentamethyldiethylenetriamine and vinylbenzyl chloride), Glycidyl methacrylate/trimethylamine, or Glycidyl methacrylate/N,N-dimethylethylenediamine reaction product.

Anion exchange membrane embodiments described herein may have a resistivity of less than about approximately 1.0 Ω-cm², for example, less than about approximately 0.5 S2-cm². In certain embodiments, the anion exchange membranes described herein may have a permselectivity of greater than about approximately 90%, for example, greater than about approximately 95%. In some embodiments, the ionogenic monomers for the production of anion exchange membranes may be or comprise Trimethylammoniumethylmethacrylic chloride crosslinked with ethyleneglycoldimethacrylate, or glycidyl methacrylate/N,N-dimethylethylenediamine reaction product crosslinked with ethyleneglycoldimethacrylate, and the crosslinked ion transferring polymer formed by polymerization of N,N,N′,N′,N″-pentamethyldiethylenetriamine di(vinylbenzyl chloride (a quaternary salt of N,N,N′,N′,N″-pentamethyldiethylenetriamine and vinylbenzyl chloride) or 1,4-diazabicyclo[2,2,2]octane di(vinylbenzyl chloride) (a quaternary salt of 1,4-diazabicyclo[2,2,2]octane (DABCO) and vinylbenzyl chloride).

Multifunctional monomers containing one or more ionic groups may be used. Without being limited by the example, monomers such as 1,4-divinylbenzene-3 sulfonic acid or its salts may be used. The degree of crosslinking may range from 2% to 60%. Multifunctional monomers suitable to provide crosslinking with monomers containing negatively or positively charged groups include as representative examples, without being limited by such examples ethyleneglycol dimethacrylate, 1,3-butanediol dimethacrylate, 1,3-butanediol diacrylate, 1,4-butanediol dimethacrylate, 1,4-butanediol diacrylate, 1,6-hexanediol diacrylate, pentaerythritol triacrylate, tetraethylene glycol dimethacrylate, divinyl benzene, trimethylolpropane triacrylate, isophorone diisocyanate, glycidylmethacrylate, trimethylolpropane trimethacrylate, ethoxylated (n) bisphenol A di(meth)acrylate (n=1.5, 2, 4, 6, 10, 30), ethoxylated (n) trimethylolpropanetri(meth)Acrylate (n=3,6,9,10,15,20), propoxylated(n) trimethylolpropane triacrylate (n=3,6), vinylbenzyl chloride, glycidyl methacrylate and the like.

The polymerization initiator may be a free radical polymerization initiator. Free radical polymerization initiators which may be employed include, for example, benzoyl peroxide (BPO), ammonium persulfate, 2,2′-azobisisobutyronitrile (AIBN), 2,2′-azobis(2-methylpropionamidine)dihydrochloride, 2,2′-Azobis[2-(2-imidazolin-2yl)propane]dihydrochloride, 2,2′-Azobis[2-(2-imidazolin-2-yl)propane], and dimethyl 2,2′-azobis(2-methylpropionate).

The substrate pore filling or saturation process may be done at a slightly elevated temperature (for example, >40° C.) to reduce air solubility. In other embodiments, the substrate pore or saturation process may be done after a mild vacuum treatment of the substrate sample submerged in the formulation solution. Substrate samples may be presoaked and then placed on a polyester or similar sheet and covered with a covering sheet. The soaked and covered substrate may be smoothed out to remove air bubbles. Several presoaked pieces may be layered and then placed on the polyester or similar sheet and covered with a covering sheet and smoothed out to remove air bubbles.

The soaked substrate may be heated in an oven at a temperature sufficient and for a time necessary to initiate complete polymerization. The soaked substrate may be placed on a heated surface at a temperature sufficient and for a time necessary to initiate and complete polymerization. Alternate methods for initiation of the polymerization reaction may be used. Ultraviolet light or ionizing radiation, such as gamma radiation or electron beam radiation may be used to initiate the polymerization reaction.

A continuous pilot or manufacturing method may comprise saturating the porous substrate, initiating and completing the polymerization, and washing or leaching out non-polymerized species from the now-formed membrane. The membrane may be optionally dried. Conditioning with a salt solution may be performed in a continuous immersion process, such as through a tank of a salt solution, or by soaking a wound-up roll of membrane, or after fabrication into a module.

If the monomer solution is formulated with a solvent which wets out the substrate, the process may start by feeding substrate from a roll into and through a tank of the monomer formulation and wiping off excess solution. The soaked substrate may be assembled between two layers of plastic sheeting fed from rolls and nipped between two rolls to remove air and produce a smooth multilayered assembly. One exemplary sheeting material is polyethylene terepthalate film. Other sheeting materials may be employed. The assembly may be processed through an oven, or over a heated roll, to initiate and complete polymerization. One alternative method may include running the saturated sheet through an oven blanketed with inert gas. The inert gas may be suitable for use with solvents having a high boiling point.

UV light initiation with suitable polymerization initiators may be used. The method may include irradiating the assembly with UV light at an intensity sufficient and for a time necessary to initiate and complete polymerization. For example, the three-layer assembly described may be run through a tunnel or other process equipment having an inlet and outlet for the substrate web with UV light sources on one or both sides of the web. With a high boiling formulation, the method may be performed in an inert gas atmosphere.

The covering sheets may be removed after polymerization. The now-formed membrane may be washed and optionally dried.

An organic solvent may be used as a reactant carrier. One useful class of solvents is dipolar aprotic solvents. Some examples of suitable solvents include dimethyl acetamide, dimethyl formamide, dimethyl sulfoxide, hexamethylphosphoramide or -triamide, acetone acetonitrile, and acetone. The organic solvent may be employed for solvating ionic group containing monomers and monomers that are not water soluble. One exemplary solvent is N-methyl pyrrolidone. Other solvents which may be employed are N-propanol and dipropylene glycol. Similar hydroxy containing solvents, such as alcohols, for example isopropanol, butanol, diols such as various glycols, or polyols, such as glycerine, may be used in certain embodiments. Other solvents are within the scope of the disclosure. The solvents discussed may be used alone or in combination. In some the solvents may be used with water to increase solubility of ionic containing organics.

The monomer mixture may be selected to engineer a cross-linked copolymer to produce a membrane having a desired balance of properties. For example, combining a water soluble and/or swellable ionogenic monomer with a non-water swelling comonomer may produce a copolymer with a high degree of ionic groups and reduced swelling in water. Such an ion exchange membrane may be used for desalination. In particular, the exemplary copolymers may have better physical strength in water and suffer less dimensional change in use due to changes in water ionic content or temperature changes. Thus, the exemplary ion exchange membranes may exhibit a suitable mechanical strength, low electrical resistance, and high counter ion permselectivity, for example, for seawater electrodialysis.

The ion exchange membranes disclosed herein may comprise a charged functionalizing layer covalently bound to the cross-linked ion-transferring polymeric layer.

Many ion exchange membranes are multivalent selective. A multivalent ion selective membrane may refer to an ion exchange membrane that selects for transport of a multivalent ion. For example, a typical cation ion exchange membrane used for ED allows a multivalent ion transport faster than a monovalent ion. The faster transport of a multivalent ion typically occurs because the higher charge number ion is attracted by a larger electrical force during migration under the same electrical field.

The ion exchange membranes disclosed herein may be monovalent selective membranes. The ion exchange membrane may be engineered to select for monovalent ions against multivalent ions by controlling charge factors, such as, surface depletion condition, membrane hydrophobicity, cross-link degree, and membrane intrinsic charge conditions. For instance, the cross-link degree and hydrophilicity modifications of a cation exchange membrane may produce a significant retard of the multivalent ion versus monovalent ions by creating a low water condition inside the membrane which is unfavorable to multivalent ions.

The monovalent selective membranes disclosed herein may have engineered surface modifications. The surface modification of the ion exchange membrane may be produced by providing charged molecules on the surface of the membrane to retard ion transport with higher valence charge number. A monovalent selective cation ion exchange membrane may have positively charged molecules on the surface. For example, a strong acid cation exchange membrane may be functionalized with sulfonic acid groups as the charged group. A weak acid membrane may be functionalized with carboxylic acid groups making up the fixed charged group. Quaternary and tertiary positively charged ammonium, respectively, may be employed to functionalize the membrane with positive charged groups in strong and weak base anion exchange membranes. A monovalent selective anion ion exchange membrane may have negatively charged molecules on the surface.

Furthermore, the strength of the surface charge repellant may be engineered by controlling charge distribution on the surface of the membrane. The strength of the charge repellent typically depends on the charge distribution when the same number of charged molecules are provided on the surface. Briefly, the electrical field strength of the ion exchange membrane is defined by dq/dx; where q is charge number or concentration and x is the depth along the ion transport.

Thus, in some embodiments, the charged functionalizing layer formed on the surface of the membrane may be selected to be a monolayer, which does not substantially affect the ion transport resistance significantly while providing a strong barrier or very large dq/dx value to multivalent ion compared to monovalent ion. Additionally, a monolayer may not significantly affect the entire conductance of the membrane. For example, a monolayer may not significantly impact the water molecule transport with the ion.

The cross-linked ion-transferring polymeric layer of the ion exchange membrane support may be functionalized by covalently binding an intermediate layer to the polymeric layer and reacting the intermediate layer with a charged functionalizing layer. In certain embodiments, the intermediate layer may be a molecule comprising an amine group. During the intermediate reaction, the amine group may exchange with water to form four covalent bonds or ammonium. The intermediate layer may comprise surface adsorbed polyethylenimine (PEI). The various primary, secondary and tertiary amines may provide a significant selectivity to the multivalent ions. The PEI molecule structure is shown in FIG. 1.

As previously described, the charged functionalizing layer may be selected to be a monolayer. In particular, the charged functionalizing layer may be a monolayer on a surface of the ion exchange membrane. Penetration of the functionalizing layer into the bulk of the membrane may react with the charged ion-transferring layer, resulting in a reduction of the membrane permselectivity function. Thus, the intermediate layer may have a size sufficient to bind to the surface of the microporous polymeric membrane coated with the cross-linked polymer, without substantially penetrating the pores of the membrane. For instance, the intermediate layer may have a size sufficient to be substantially inhibited from penetrating the micropores of the polymeric substrate.

The intermediate layer may be selected to have a size greater than the pores of the microporous polymeric substrate. Thus, in some embodiments, the intermediate layer may comprise a molecule having a molecular weight of at least 100 g/mol, for example, at least 600 g/mol. The intermediate layer may comprise a molecule having a molecular weight of at least 1,000 g/mol, for example, at least 10,000 g/mol. The intermediate layer may comprise a molecule having a molecular weight of at least 40,000 g/mol, for example, at least 50,000 g/mol or at least 60,000 g/mol. The intermediate layer may comprise a molecule having a molecular weight of at least 70,000 g/mol, at least 80,000 g/mol. The intermediate layer may comprise a molecule having a molecular weight of between 60,000 g/mol and 120,000 g/mol. In exemplary embodiments, the intermediate layer may comprise branched PEI. The branched PEI may have a molecular weight as described herein.

Conventionally, PEI may be coated on a surface of a cation exchange molecule by physisorption. Briefly, physisorption is a physical adsorption reaction that leads to an ionic bond of PEI on the cross-linked polymeric layer, as shown in FIG. 2. However, the ionic bond is generally not stable, such that the surface charged molecule may dissolve in the water leading to a loss of selectivity.

The methods disclosed herein may comprise attaching the intermediate layer to the cross-linked ion-transferring polymeric layer by chemisorption. Chemisorption may generally include chemically adsorbing the intermediate layer to the polymeric layer, such that the intermediate layer is bound by a covalent bond. Thus, the ion exchange membrane supports disclosed herein may have an intermediate layer bound by a covalent bond. The covalent bond may provide increased surface stability for the ion exchange membrane. As a result, the covalent bond may increase selectivity of the membrane for a longer lifespan. In some embodiments, the ion exchange membrane may have an operational lifespan of more than 150 days, for example, more than 400 days in use at room temperature. The ion exchange membrane may have an operational lifespan of more than 2 years or more than 3 years in use at room temperature. Additionally, the ion exchange membrane may have an operational lifespan of more than 30 days in use at 80° C.

The intermediate layer may have an attachment group configured to covalently bind the intermediate layer to the cross-linked ion-transferring polymeric layer. The attachment group may be selected to provide increased stability. For instance, the attachment group may be selected to provide a bond that is sufficiently stable to withstand organic compounds in the water to be treated in use. In particular, the attachment group may be sufficiently stable to withstand organic contaminants such as benzyne, toluene, ethylbenzene, and xylene for extended periods of time while in use. Thus, the ion exchange membranes disclosed herein may be used to treat wastewater comprising organic contaminants, such as, produced water, ground water, brackish water, brine, and seawater. The wastewater may comprise, for example, between about 100-1000 ppm of TDS. In certain embodiments, the wastewater may comprise, for example, between about 100-400 ppm TDS, between about 400-600 ppm TDS, or between about 600-1000 ppm TDS.

The monovalent selective cation exchange membranes disclosed herein may be used to treat water comprising at least one hardness ion. For instance, the water to be treated may comprise at least one positively charged divalent ion. In certain embodiments, the water to be treated may comprise at least one hardness ion selected from Ca²⁺ and Mg²⁺. Additionally, the monovalent selective cation exchange membranes disclosed herein may be used for agricultural water treatment, where use of water with a high sodium content can damage soil, but magnesium and calcium are beneficial.

In one exemplary embodiment, the attachment group may be a styrene group. Chemisorbing the amine intermediate layer to the ion cross-linked polymeric layer may include plasma grafting the amine intermediate layer to the surface.

The chemisorption of the intermediate layer may be performed in a multi-step method. In one exemplary embodiment, as shown in FIG. 3, the amine group may react with sulfonyl chloride to form a stable immobilized amine group in a series of reactions. Briefly, the method of producing the ion exchange membrane may comprise covalently binding a styrene layer to the cross-linked polymeric layer to form a first intermediate layer. The styrene layer may comprise a sulfonyl chloride group. The reaction may be performed for an amount of time sufficient to bind the styrene layer to the bulk of the substrate. For instance, the reaction may be performed for an amount of time sufficient for the styrene layer to penetrate the pores of the substrate. The amount of time sufficient may be on an order of hours, particularly in embodiments wherein the substrate has a thickness of less than about 155 μm, for example, less than about 25 μm. For example, the reaction may be performed in less than about 10 hours. The reaction may be performed in about 1-2 hours, about 2-5 hours, about 3-6 hours, or about 4-7 hours.

In exemplary embodiments, the styrene layer may comprise divinylbenzene (DVB). In such embodiments, the method may further comprise attaching a sulfonyl chloride group to the DVB styrene layer. To attach the sulfonyl chloride group to the DVB styrene layer, the method may comprise polymerizing and chlorosulfonating the DVB. In exemplary embodiments, the chlorosulfonation may be performed by fuming concentrated sulfuric acid or chlorosulfonic acid (ClSO₃H) on the DVB. The chlorosulfonic acid may be hydrolyzed with caustic solution. In such exemplary embodiments, the chlorosulfonation reaction attaches a ClSO₂ group to the DVB.

The chlorosulfonation reaction may be performed for an amount of time sufficient to penetrate the bulk of the substrate. The amount of time sufficient for the chlorosulfonation reaction may be on an order of hours. For example, the chlorosulfonation reaction may be performed in less than about 10 hours. The chlorosulfonation reaction may be performed in about 1-2 hours, about 2-5 hours, about 3-6 hours, or about 4-7 hours.

The method of producing the ion exchange membrane may comprise aminating the sulfonyl chloride group of the first intermediate layer with an amine group layer to produce a chemically immobilized amine containing group on a surface of the membrane support. The amine group may comprise a primary or secondary amine. The chemically immobilized amine group may generally comprise a functionalizable amine. The functionalizable amine may be selected based on the designed charged molecule. Furthermore, the amine group may have a size sufficient to bind an exterior surface of the substrate, while being substantially inhibited from penetrating the pores of the substrate. The amination reaction may be performed overnight. For example, the amination reaction may be performed for an amount of time of between about 10-18 hours. The chemically immobilized ammine containing group may be PEI or branched PEI, as previously described.

The method may comprise functionalizing the ion exchange membrane support by reacting the surface intermediate layer with the charged functionalizing layer. For instance, the method may comprise binding a charged functionalizing group to the chemically immobilized amine layer. Any of the charged functionalizing molecules described above may be attached to the membrane support. In certain embodiments, for example, to produce a cation exchange membrane, the method may comprise hydrolyzing PEI with sulfonic acid group, for example, sulfonyl hydroxide. The produced cation exchange membrane will generally have the charged functionalizing layer covalently bound to the ion exchange membrane support. The covalent bond may provide greater selectivity and stability of the ion exchange membrane in use, as previously described.

The monovalent selective ion exchange membranes disclosed herein may have a counter ion permselectivity of at least 100%. For example, the monovalent selective ion exchange membranes disclosed herein may have a counter ion permselectivity of between about 100%-105% or between about 100%-103%. The monovalent selective ion exchange membranes disclosed herein may have an initial selectivity of 8 to 12 fold Na/Ca (ppm) at room temperature. The monovalent selective membranes disclosed herein may have a resistivity of less than about 7 Ω-cm², for example, less than about 5 Ω-cm², between about 2-7 Ω-cm², or between about 3-5 Ω-cm².

The function and advantages of these and other embodiments can be better understood from the following examples. These examples are intended to be illustrative in nature and are not considered to be limiting the scope of the invention.

EXAMPLES Example 1: Production of Cation Exchange Membrane Test Coupons

The following laboratory method was used to investigate formulation and process effects by producing small coupons for resistivity and counter ion permselectivity testing. Porous membrane substrate 43 mm diameter coupons were die cut. Somewhat larger discs (50 mm or 100 mm diameter) of transparent polyester sheets were also die cut. A 105 mm aluminum weighing boat was used to hold a set of coupons. The coupons were sandwiched between two polyester film discs.

First, substrate coupons were thoroughly wetted with a monomer solution to make up a template. This was done by adding the formulated solution to the aluminum boat, and immersing a polyester film disc with a substrate coupon layered on it into the solution so that the porous support is saturated. The saturated support was then removed from the monomer solution and placed on a piece of polyester film. Air bubbles were removed from the coupon by, for example, smoothing or squeezing the coupon with a convenient tool, such as a small glass rod, or by hand. A second polyester disc was then layered on top of the first coupon and smoothed to have complete surface contact between the coupon and the lower and upper polyester film layers. A second porous substrate was then layered on the upper polyester film and the saturation, smoothing and addition of a over layer of polyester film repeated to give a multilayer sandwich of two coupons and three protective polyester film layers. A typical experimental run would have a multilayered sandwich of 10 or more saturated substrate coupon layers. The rim of the aluminum boat was crimped down to hold the disc/coupon assembly, if required.

The sample containing the boat and coupon assembly was placed into an oven at 80° C. for up to 30 minutes. The bag was then removed and cooled, and the now reacted cation exchange membrane coupons were placed in 0.5N NaCl solution at 40° C.-50° C. for at least 30 minutes, with NaCl soak of up to 18 hours being found satisfactory.

The described method was suitable to prepare the cation exchange membrane test coupons.

Example 2: Monovalent Selectivity of the Cation Exchange Membrane

To evaluate the selectivity between the monovalent and multivalent ions, a solution containing 0.15 M NaCl and 0.15 CaCl₂ was used to feed the dilute compartment. The concentrate and the two electrodes were fed with a 0.30 M KNO₃ solution. The dilute stream was a 150 ml sample reservoir with a total volume of about 75 ml. The concentrate stream (0.3M KNO₃) was a 1000 ml solution to ensure a negligible concentration increase. Typically a 25% salt removal can be reached at 70 mA for a 7 cm² membrane sample with an experiment time of 3 hours.

The current density was 100 A/m². All three streams were circulated by 3 peristaltic pumps, each having a nominal pumping speed of 200 ml/min. The dilute stream was sampled for ion chromatography (IC) analysis. Each sample taken was 100.0 μl, and diluted to 50 ml for analysis. Typically, 4-6 samples were taken throughout each membrane experiment. The sample removal did not affect the total volume of the dilute stream. In most cases, due to the insignificant concentration difference between the concentrate and dilute stream, the water loss was minor. Volume adjustment was not required for the IC analysis samples.

Conventional Cation Exchange Membrane

FIGS. 4A-4B are graphs of the molar quantity of Ca²⁺ and Na ions (mol/L) in the dilute stream over time (seconds) for desalt using two conventional membranes, as described in the experimental procedures above. The graphs show selectivity of Ca/Na (mol/L). The molar transport ratio between Ca²⁺ and Na⁺ was around 2 for the conventional cation exchange membranes. The result is mainly due to the charge effect. Ca²⁺ migrates in the electrical field through membrane faster than Na due to the greater charge of the ion. However, both Ca²⁺ ions and Na⁺ ions were steadily removed, as shown by the slopes of the line.

Monovalent Selective Cation Exchange Membrane

A monovalent selective cation exchange membrane prepared by the methods disclosed herein (for example, as described in example 5 below) with a PEI having a molecular weight of 600 g/mol was tested as described above. The results are shown in the graph of FIG. 5. Briefly, the permselectivity of Ca²⁺/Na⁺ was 11. Thus, Ca²⁺ is 22 times retarded in transport as compared to the unmodified conventional membranes described above. Accordingly, the monovalent selective cation exchange membranes described herein provide an increased permselectivity as compared to conventional cation exchange membranes.

Example 3: Preparation of Membrane Test Coupons

A membrane was prepared by soaking a porous polyethylene (PE) film (having a thickness of 24 or 34 μm) in a styrene (ST)/divinylbenzene (DVB)/N-Methyl-2-Pyrrolidone (NMP) solution for 0.01-4 hours. The mixture had a polymerization initiator added and a composition pf ST:DVB:NMP of 7:1:2 (by mass). The PE film was saturated with the solution and placed between two mylar sheets. Air bubbles between the mylar sheets were removed. More solution was added to avoid any “white area” due to evaporation of the solution after extended exposure. The membrane was heated to about 80-90° C. for 1-4 hours. Typical membrane dimensions for such an experiment are 4×15 inches.

The membranes so prepared were cut into 1.5 inch disc coupons. The coupons were soaked in ClSO₃H/CH₃Cl solution having a composition of ClSO₃H:CH₃Cl of 1:2 (by volume) for 24 hours at a temperature of 4° C. The membrane was removed from the solution and rinsed with NMP and methanol. The rinsed membrane was then dried with a napkin and deemed ready for subsequent treatment and testing.

The resistance of such membrane is typically 2500 Ω-cm² and no permselectivity. The resistance reported was beyond the measurement of the instrument.

Example 4: Preparation of Cation Exchange Membrane from the Membrane Test Coupons of Example 3

The membrane test coupons of example 3 were treated to produce cation exchange membrane test coupons.

After drying with the napkin the membrane was placed in a 1N NaOH solution for about 15 minutes. The membrane was removed from the NaOH solution and rinsed with water and conditioned in a 0.5M NaCl solution.

The membrane had a resistance of between 1.8-3 Ω-cm² and a counter ion permselectivity 101%-104%.

Example 5: Surface Modification of the Cation Exchange Membrane Test Coupons of Example 4

The cation exchange membrane test coupons of example 4 were functionalized to prepare monovalent and multivalent selective cation exchange membrane test coupons.

After drying with the napkin the membrane was placed in a PEI water solution overnight (about 15 hours). It was tested that the PEI solution may have a pH between 8-12.6. The membrane was removed from the PEI solution and rinsed with water. The membrane was soaked in a 1N NaOH solution for 15-22 minutes, to ensure the substrate bulk SO₂Cl groups were converted to SO₃Na completely.

The membrane surface was modified with PEI polymer molecule and subject to various tests. The membrane had a resistance of between 2.8-7 Ω-cm² and a counter ion permselectivity 100%-103%.

Example 6: Modification of Ground Water

A sample water containing Na at 800 ppm, Ca²⁺ at 250 ppm, and Mg²⁺ at 50 ppm was prepared as a representative ground water. In practice, ground water has a vast variation of the three cations. The composition tested herein was an average value.

The sample ground water was treated with the monovalent selective cation exchange membrane described in example 5 and a conventional cation exchange membrane described in example 3. The results are shown in the graphs of FIGS. 6A-6B. Concentration of Na⁺, Ca²⁺, and Mg²⁺ ions were measured. Sodium adsorption ratio (SAR) was also measured. SAR is an important index for the water hardness requirement of water used for irrigation.

Briefly, the results show the monovalent selective membrane of example 5 can reduce SAR value of the treated water to 3. By comparison, the cation exchange membrane of example 3 removes all ions, increasing SAR value due to the removal of the multivalent ions. Accordingly, the monovalent selective membranes described herein may decrease SAR value of treated ground water.

Example 7: Treatment of Seawater

The monovalent selective cation exchange membrane as described in example 5 was used to treat seawater for hardness removal. The removal of hardness in seawater may be important for many processes such as hypochlorite generation, oil extraction, and table salt production. The results are presented in FIG. 7. Specifically, the change of concentration of Mg²⁺, Ca²⁺, and Na ions in the dilute stream over time is shown in the graph of FIG. 7. Briefly, the concentration of Mg²⁺ and Ca²⁺ remains relatively constant, while the concentration of Na ions is reduced. Accordingly, the monovalent selective membranes described herein may be used to reduce Na ion concentration in seawater.

Example 8: Stability of the Monovalent Selective Cation Exchange Membrane

The monovalent selective cation exchange membrane of example 5 was soaked in a 0.5 M NaCl solution at room temperature. A conventional cation exchange membrane having physiosorbed PEI was also tested. FIG. 8 is a graph of the change in membrane permselectivity after time. Briefly, after 150 days of soaking, the monovalent selective membrane has a permselectivity of greater than 9 for Na ions versus Ca²⁺ ions. The monovalent selective membrane has a higher selectivity than commercially available conventional product, and also shows significant stability over time. Accordingly, the monovalent selective membrane has better selectivity and a longer service life than a conventional membrane, and remains stable after an extended period of use.

Example 9: Monovalent Selective Cation Membrane Performance Study

Monovalent selective cation membrane performance was studied under laboratory conditions using membrane coupons with a surface area of 7 cm². Selectivity was determined using a lab ED module (as shown in FIG. 9) containing a diluting and concentrate compartment. The solutions in these compartments were circulated independently via peristatic pumps as well as a K₂SO₄ electrolyte circulating through the anodic and cathodic compartments. The dilute stream had a total volume of about 75 ml and its ion constituents were monitored by ion chromatography (IC). Both the cation exchange membrane and the anion exchange membrane used in testing had a high co-ion exclusivity with a 98% preferential transport of counterions. Current densities were chosen to avoid operating beyond limiting current.

A synthetic ground water composition (having 800 ppm Na⁺, 260 ppm Ca²⁺, 76 ppm Mg²⁺) was used to test the selectivity of the monovalent selective cation exchange membrane at a current density of 30 A/m². FIGS. 10A-10B show the concentrations of target cations in the dilute compartment over time. FIG. 10B shows all cation concentrations decreasing by passing through non-selective membrane, while FIG. 10A shows only the Na being diluted by the monovalent selective cation exchange membrane. FIGS. 10C-10D show the concentration of target cations in mol/L in the dilute compartment over time.

Sea salt recovery is also demonstrated in an experiment. The dilute compartment contains an initial solution with the major ions of seawater (having 17000 ppm 2800 ppm SO₄ ²⁻, 9000 ppm Na⁺, 1200 ppm Mg²⁺ and 300 ppm Ca²⁺) diluted to a TDS of 500 ppm. FIGS. 11A-11B show the concentration of select ions in the concentrate compartment using a monovalent selective anion exchange membrane and a monovalent selective cation exchange membrane with an applied current density of 300 A/m². The blue squares represent the concentrations of the major ions in raw sea water.

The graphs clearly demonstrate the increase in concentration of chloride over sulfate (FIG. 11A) and sodium over calcium (FIG. 11B) in the concentrate compartment over time. The combination of the monovalent selective anion and cation exchange membranes demonstrates the applicability to recover sea salt from seawater using an ED process having both membranes. Moreover, non-selective and monovalent selective membrane cell pairs can be combined to produce specifically targeted ionic compositions in EDR product water.

Comparison of the initial selectivity and lifetime selectivity (stability) of a conventional/commercially available monovalent selective membrane and the monovalent selective membrane disclosed herein is shown in FIGS. 12A-12B. The selectivity in FIGS. 12A-12B is expressed in fold change of sodium ion concentration over calcium ion concentration, on a parts per million (ppm) or molar (M) concentration scale.

The conventional/commercially available membrane was produced by a method including physisorption of PEI. FIG. 12A shows membrane selectivity over soak time in a 0.5 M NaCl solution at a temperature of 80° C. The results were extrapolated by using a temperature correction which was previously derived from experiment with an Arrhenius plot with a slope of 2.5/10° C. As shown in the accelerated testing, the loss of selectivity with the monovalent cation selective membranes disclosed herein is considerably reduced over time compared to the conventional membrane. Furthermore, by extrapolating lifetime from high temperature to normal operation temperature (as shown in FIG. 12B), an acceptable lifetime for the monovalent selective cation exchange membrane disclosed herein is determined with a high selectivity of monovalent cations over divalent cations.

Example 10: Uses of the Monovalent Selective Cation Exchange Membrane

Examples of how the monovalent selective cation exchange membranes disclosed herein can be used in water treatment systems are described in the following examples. EDR product and reject water qualities were modeled using in-house finite element analysis (FEA) projection software for monovalent selective cation exchange membranes. Scaling indices (SI) for the reject waters were calculated using the PHREEQC software (a computer program written in the C++ programming language that is designed to perform a wide variety of aqueous geochemical calculations) at various instantaneous EDR recoveries. The results were compared to field data from non-selective EDR installations as well as FEA models.

Application 1: Industrial Water Brine Minimization

Many industrial applications use reverse osmosis (RO) to produce low salinity water. Often, the RO systems are limited to lower recoveries because of potential scale formation, but brine disposal can be a costly portion of the overall process. Monovalent selective cation exchange membrane EDR can be used to treat the brine to achieve discharge limits, greatly reducing the disposal costs.

One comparative application site operates an RO at 75% recovery with a rejected stream TDS of 2297 mg/L. Without further treatment, 25% of the total feed flow would need to be disposed of as brine waste. By employing non-selective EDR, this brine can be reduced to 5.7% of the feed flow. The monovalent selective cation exchange membranes can further reduce the brine waste to 3.2% of feed flow by increasing the recovery of the EDR process from 82% to 90% before the danger of precipitating CaCO₃ scale. Table 1 shows the major ion concentrations for the streams from field testing and monovalent selective cation exchange membrane modeling as well as the SI for CaCO₃ at the maximum EDR recovery.

TABLE 1 Industrial brine minimization stream analysis Non- Non- Monovalent Monovalent Selective Selective selective selective Product Reject Product Reject Feed 82% Recovery 82% Recovery 90% Recovery 90% Recovery CaCO₃ 0.5 −1.54 2.3 −0.55 2.27 Scaling Index TDS (mg/L) 2297 526 10369 526 18253 pH (su) 7.98 7.11 8.62 7.11 9.02 Ca (mg/L) 100.9 6.7 564.6 93.2 199.0 Mg (mg/L) 46.6 3.2 358.6 43.4 89.3 Na (mg/L) 638.8 165.9 1820.0 19.9 6223.3 K (mg/L) 24.2 6.4 110.4 0.9 234.3 HCO₃ (mg/L) 350.8 115.9 1079.5 87.8 2748.5 Cl (mg/L) 1013.1 182.1 5413 253.7 7937.9 NO₃ (mg/L) 4.9 0.6 13.6 1.2 38.7 PO₄ (mg/L) 5.4 2.4 21.0 1.4 42.6 SO₄ (mg/L) 92.8 33.3 281.0 23.2 727.1

Application 2: Produced Water Discharge

Processes used to harvest oil and gas can also generate “produced water” that is a challenge to treat for environmental discharge. An EDR system was piloted at a produced water facility where desalination is a major component of the treatment process. The sample water is a water with high concentrations of silica that limit pressure driven membrane recoveries.

The EDR pilot study demonstrated 88% instantaneous recovery while reducing TDS from 8587 mg/L to 2107 mg/L in the first stage of a two-stage process, but could not achieve higher recovery due to the potential formation of BaSO₄ scale. By applying the selectivity from the monovalent selective cation exchange membrane, the same TDS reduction can be achieved while operating at an expected recovery of 98%. Table 2 shows the projected product and concentrate stream analyses at 97% recovery.

TABLE 2 Produced water stream analysis Non- Non- Monovalent Monovalent selective selective selective selective Product Reject Product Reject Feed 88% Recovery 88% Recovery 97% Recovery 97% Recovery BaSO₄ 0.82 −0.64 1.94 0.59 1.59 Scaling Index TDS (mg/l) 8587 2107 54881 2107 218164 pH (su) 7.6 6.9 8.0 6.9 8.68 Ca (mg/l) 20.4 2.8 147.4 16.4 97.1 Mg (mg/l) 3.8 0.3 27.9 3.1 17.7 Na (mg/l) 2670 726 17833 646 68633 K (mg/l) 25.0 7.0 148.5 5.3 670.4 Ba (mg/l) 4.7 0.3 32.5 3.9 19.7 HCO₃ (mg/l) 3743 911 19157 921 95671 Cl (mg/l) 2033 414 17003 500 51965 NO₃ (mg/l) 4.7 6.9 7.2 1.2 119.6 SO₄ (mg/l) 34.1 6.4 389.7 8.4 870.8 SiO₂ (mg/l) 24.0 22.7 27.2 22.7 27.2

Application 3: Agricultural Desalination

To reduce the load on freshwater sources in agricultural applications, alternative supplies with brackish water qualities should be considered. While some crops, like barley and cotton, are more tolerant to saline water conditions, constant use of a brackish water will generally accumulate salt in the soil and negatively affect yield without proper leaching through the addition of freshwater. For more salt-sensitive crops, including fruit plants, even greater care must be taken, and desalination is often required.

In addition to the overall salt content, cation concentrations can have varying effects on soil structural stability. The effect can be expressed by the sodium adsorption ratio (SAR) and the cation ratio of structural stability (CROSS). While the effect these parameters can have on agricultural yield depends on the specific crop and salinity, a lower SAR or CROSS value typically indicates better soil stability. The equations for SAR and CROSS are shown below:

$\begin{matrix} {{SAR} = \frac{\lbrack{Na}\rbrack}{\left( {\lbrack{Ca}\rbrack + \lbrack{Mg}\rbrack} \right)^{0.5}}} & (1) \\ {{CROSS} = \frac{\lbrack{Na}\rbrack + {0.56\lbrack K\rbrack}}{\left( {\lbrack{Ca}\rbrack + {0.6\lbrack{Mg}\rbrack}} \right)^{0.5}}} & (2) \end{matrix}$

The monovalent selective cation exchange membrane EDR may selectively remove sodium and potassium over calcium and magnesium. As a result, it may be suited for reducing TDS for agricultural applications and maintaining a low SAR value. In particular, monovalent selective cation exchange membrane EDR maintains low SAR value throughout the range of product TDS concentrations with low energy and no additional process steps, as required by many crops.

A sample brackish feed water was modeled with non-selective EDR and monovalent selective EDR with the same product TDS. The product ion concentrations are presented in Table 3. Monovalent selective cation exchange product water has a SAR value of 0.16 compared to 6.59 with a non-selective process.

TABLE 3 Feed and product water analyses for sample brackish agricultural water. Feed Non-Selective Product MSCEM Product TDS (mg/L) 5804.6 529.2 556.2 Ca (mg/L) 540.3 3.9 170.9 Mg (mg/L) 327.3 16.4 3.9 Na (mg/L) 792.3 133.5 7.8 K (mg/L) 3.1 1.7 1.0 Sr (mg/L) 8.4 5.0 7.7 SO₄ (mg/L) 3130.0 199.5 209.0 HCO₃ (mg/L) 413.9 147.1 132.0 Cl (mg/L) 582.1 19.5 21.8 Fl (mg/L) 0.4 0.0 0.0 NO₃ (mg/L) 6.9 2.5 2.0 SAR 6.62 6.59 0.16 CROSS 7.42 8.20 0.17

Thus, the monovalent selective cation exchange membranes disclosed herein are suitable for such applications as industrial water brine minimization, produced water discharge, and agricultural desalination.

The phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. As used herein, the term “plurality” refers to two or more items or components. The terms “comprising,” “including,” “carrying,” “having,” “containing,” and “involving,” whether in the written description or the claims and the like, are open-ended terms, i.e., to mean “including but not limited to.” Thus, the use of such terms is meant to encompass the items listed thereafter, and equivalents thereof, as well as additional items. Only the transitional phrases “consisting of” and “consisting essentially of,” are closed or semi-closed transitional phrases, respectively, with respect to the claims. Use of ordinal terms such as “first,” “second,” “third,” and the like in the claims to modify a claim element does not by itself connote any priority, precedence, or order of one claim element over another or the temporal order in which acts of a method are performed, but are used merely as labels to distinguish one claim element having a certain name from another element having a same name (but for use of the ordinal term) to distinguish the claim elements.

Having thus described several aspects of at least one embodiment, it is to be appreciated various alterations, modifications, and improvements will readily occur to those skilled in the art. Any feature described in any embodiment may be included in or substituted for any feature of any other embodiment. Such alterations, modifications, and improvements are intended to be part of this disclosure and are intended to be within the scope of the invention. Accordingly, the foregoing description and drawings are by way of example only.

Those skilled in the art should appreciate that the parameters and configurations described herein are exemplary and that actual parameters and/or configurations will depend on the specific application in which the disclosed methods and materials are used. Those skilled in the art should also recognize or be able to ascertain, using no more than routine experimentation, equivalents to the specific embodiments disclosed. 

What is claimed is:
 1. A monovalent selective ion exchange membrane, comprising: a polymeric microporous substrate; a cross-linked ion-transferring polymeric layer on a surface of the substrate; and a charged functionalizing layer covalently bound to the cross-linked ion-transferring polymeric layer.
 2. The monovalent selective ion exchange membrane of claim 1, wherein the membrane has a total thickness of about 20 μm to about 155 μm.
 3. The monovalent selective ion exchange membrane of claim 2, wherein the membrane has a total thickness of about 25 μm to about 55 μm.
 4. The monovalent selective ion exchange membrane of claim 1, being a cation exchange membrane wherein the charged functionalizing layer is a positively charged functionalizing layer.
 5. The monovalent selective ion exchange membrane of claim 4, wherein the positively charged functionalizing layer comprises at least one of a sulfonic acid group, a carboxylic acid group, a quaternary ammonium, and a tertiary amine group hydrolyzed into a positively charged ammonium.
 6. The monovalent selective ion exchange membrane of claim 1, being an anion exchange membrane wherein the charged functionalizing layer is a negatively charged functionalizing layer.
 7. The monovalent selective ion exchange membrane of claim 1, having a counter ion permselectivity of at least 100%.
 8. The monovalent selective ion exchange membrane of claim 1, having an initial selectivity of 8 to 12 fold Na/Ca (ppm) at room temperature.
 9. The monovalent selective ion exchange membrane of claim 1, having a resistivity of less than about 5 Ω-cm².
 10. The monovalent selective ion exchange membrane of claim 1, wherein the polymeric microporous substrate comprises at least one of high-density polyethylene (HDPE) and ultrahigh molecular weight polyethylene (UHMWPE).
 11. A monovalent selective cation exchange membrane support, comprising: a polymeric microporous substrate; a cross-linked ion-transferring polymeric layer on a surface of the substrate; and an intermediate layer comprising an amine group covalently bound to the cross-linked ion-transferring polymeric layer.
 12. The monovalent selective cation exchange membrane support of claim 11, wherein the intermediate layer comprises a primary amine group or a secondary amine group.
 13. The monovalent selective cation exchange membrane support of claim 11, wherein the intermediate layer comprises polyethylenimine (PEI).
 14. The monovalent selective cation exchange membrane support of claim 11, wherein the intermediate layer comprises a branched PEI having a molecular weight of at least 600 g/mol.
 15. The monovalent selective cation exchange membrane support of claim 11, wherein the intermediate layer is covalently bound to the cross-linked ion-transferring polymeric layer by a styrene group.
 16. The monovalent selective cation exchange membrane support of claim 15, wherein the styrene group is chemically bound to chlorosulfonated divinylbenzene (DVB).
 17. A method of producing a monovalent selective cation exchange membrane, comprising: chemically adsorbing a styrene intermediate layer to a cross-linked ion-transferring polymeric layer on a surface of a polymeric microporous substrate; chlorosulfonating the styrene intermediate layer to attach a sulfonyl chloride group layer to the surface of the polymeric microporous substrate; aminating the sulfonyl group layer to attach an amine group layer to the surface of the polymeric microporous substrate; and functionalizing the amine group layer with a charged compound layer to produce the monovalent selective cation exchange membrane.
 18. The method of claim 17, comprising chemically adsorbing styrene DVB to the cross-linked ion-transferring polymeric layer on a surface of a polymeric microporous substrate.
 19. The method of claim 18, comprising chlorosulfonating the styrene DVB with chlorosulfonic acid (ClSO₃H) to attach a ClSO₂ group to the styrene DVB.
 20. The method of claim 19, comprising aminating the ClSO₂ with PEI.
 21. The method of claim 20, comprising aminating the ClSO₂ with branched PEI having a molecular weight of at least 600 g/mol.
 22. The method of claim 17, comprising functionalizing the amine group layer with a positively charged group.
 23. The method of claim 22, comprising functionalizing the amine group layer with a positively charged ammonium.
 24. The method of claim 17, further comprising soaking the polymeric microporous substrate with a solution comprising an ionogenic monomer, a multifunctional monomer, and a polymerization initiator to produce the cross-linked ion-transferring polymeric layer.
 25. A water treatment system, comprising: a source of water to be treated; an electrochemical separation device fluidly connected to the source of water to be treated and comprising at least one monovalent selective cation exchange membrane having a charged functionalizing layer covalently bound to a surface of the cation exchange membrane; and a treated water outlet fluidly connected to the electrochemical separation device.
 26. The water treatment system of claim 25, wherein the source of water to be treated comprises at least one hardness ion selected from Ca²⁺ and Mg²⁺.
 27. The water treatment system of claim 25, wherein the charged functionalizing layer is a positively charged functionalizing layer comprising at least one of a sulfonic acid group, a carboxylic acid group, a quaternary ammonium, and a tertiary amine group hydrolyzed into a positively charged ammonium.
 28. The water treatment system of claim 27, wherein the charged functionalizing layer is covalently bound to the surface of the cation exchange membrane by a chemically adsorbed branched PEI layer.
 29. A method of facilitating water treatment with an electrochemical separation device, comprising: providing a monovalent selective cation exchange membrane having a charged functionalizing layer covalently bound to a surface of the cation exchange membrane; and instructing a user to install the monovalent selective cation exchange membrane in the electrochemical separation device.
 30. The method of claim 29, comprising instructing the user to fluidly connect the electrochemical separation device to a source of water to be treated comprising at least one hardness ion selected from Ca²⁺ and Mg²⁺.
 31. The method of claim 29, wherein providing the monovalent selective cation exchange membrane comprises: providing a monovalent selective cation exchange membrane support having a polymeric microporous substrate with an amine group layer covalently bound to a surface of the polymeric microporous substrate; and instructing a user to functionalize the amine group layer with a charged compound layer to produce the cation exchange membrane.
 32. A monovalent selective cation exchange membrane, comprising: a polymeric microporous substrate and a positively charged functionalizing layer covalently bound to a surface of the polymeric microporous substrate, the monovalent selective cation exchange membrane having an initial selectivity of 8 to 12 fold Na/Ca (pm) at room temperature.
 33. The monovalent selective cation exchange membrane of claim 32, having a selectivity of 4 to 8 fold Na/Ca (ppm) after 400 days in 0.5M NaCl at room temperature.
 34. The monovalent selective cation exchange membrane of claim 32, having an initial selectivity of 10 to 40 fold Na/Ca (molar) at 80° C.
 35. The monovalent selective cation exchange membrane of claim 34, having a selectivity of 3 to 6 fold Na/Ca (molar) after 30 days in 0.5M NaCl at 80° C. 